Commercially used plastic resins are lighter than glasses and have good physical properties such as moldability and impact resistance. The surfaces of many plastic resins, however, have poor scratch resistance. In order to improve the scratch resistance of plastic resins, a hard coating method has been widely used. The hard coating method includes the steps of coating a surface of an injection-molded resin with an organic-inorganic hybrid material and curing the organic-inorganic hybrid material using ultraviolet radiation. The additional coating process step, however, increases processing times and manufacturing costs and raises environmental concerns.
There is an increased need for non-coated resin products in view of the environmental problems and manufacturing costs associated with the production of coated resin products. Exterior parts of home appliances such as TVs, washing machines, and the like, and electronic products such as computers, mobile phones, and the like, are currently manufactured using non-coated resin. Surfaces of products made of non-coated resin can, however, be easily scratched by dust, cleaning outfits, impact, and the like, during storage, transport or usage thereof. Accordingly, there is a need to solve such problems by improving the scratch resistance of plastic resins and developing an evaluation method which can exactly estimate scratch resistance of a resin injection-molded product, film, or similar product.
Generally current methods for evaluating scratch resistance of a resin injection-molded product, film, coating, and the like include 1) a pencil hardness test, 2) a scratch evaluation method using a diamond or sapphire chip and 3) a rubbing test.
The pencil hardness test is a method for evaluating scratch resistance which uses pencil leads with different degrees of hardness, as shown below, to scratch a surface of a resin. The pencil hardness test determines whether a resin passes or fails the test (“Pass/Fail”) on the basis of visual estimation.
                                          6            ⁢            B                    -                      5            ⁢            B                    -                      4            ⁢            B                    -                      3            ⁢            B                    -                      2            ⁢            B                    -          B          -          HB          -                                              F          -          H          -                      2            ⁢            H                    -                      3            ⁢            H                    -                      4            ⁢            H                    -                      5            ⁢            H                    -                      6            ⁢            H                                          Softer      ⁢                          ←              --                  --                      --                          --                              --                                  --                  --                                                                          ->                  ⁢    Harder  
In the pencil hardness test, evaluation results of scratch resistance are affected by test conditions such as temperature and humidity, and the state of the pencil leads used for the test. Additionally, reproducibility of the pencil hardness test is low and the evaluation results vary from person to person, because determining whether a scratch is produced on a test sample is visually determined and producing the scratch with a constant load is difficult. Moreover, since measurement results are represented as an attribute data such as Pass/Fail, the pencil hardness tests can not provide quantitative test results. Further, the pencil hardness test cannot discriminate differences in scratch resistances between two test samples if the two test samples have the same pencil hardness grade.
The pencil hardness test is a system that statistically determines Pass/Fail. In particular, the pencil hardness test starts with the hardest pencil and continues using progressively softer pencils until the hardest pencil that does not scratch the test sample has been established. Therefore, the pencil hardness test takes a long time due to the repeated measurements.
In the scratch evaluation method using the diamond or sapphire chip, a diamond or sapphire chip is used to scratch a surface of a test sample with a load which progressively increases up to about 1 N to about 5 N. Scratch resistance of the test sample is evaluated based on a critical load at which the scratch occurs.
As in the pencil hardness test, in the scratch evaluation method using the diamond or sapphire chip, the critical load is visually checked, and the scratch resistance is evaluated based on Pass/Fail criterion. Therefore, test results of this method are difficult to objectify and quantify.
A taber test and a rubbing test are mainly used as the rubbing test.
In the taber test, the original weight of a test sample is measured. The test sample is then brought into contact with an abrasion wheel under a load of about 1000 g and allowed to spin for about 500 revolutions. After spinning, a final weight of the test sample is taken, and the weight loss of the test sample is calculated. Wear resistance and scratch resistance are evaluated using the weight loss. During the taber test, surface melting of the test sample and adhesion between the test sample and the abrasion wheel can cause errors, and thus discrimination of the taber test falls.
In the rubbing test, an abrader repeatedly scratches a surface of a test sample with a constant load at a constant rate, and the scratches formed on the surface are visually evaluated.
As described above, conventional methods for evaluating scratch resistance determine whether a sample passes or fails based on visual observation of a scratch produced on a surface of a test sample or evaluate scratch resistance based on weight loss of the test sample.
In such conventional methods, scratch resistance of the same test sample tends to vary from measurer to measurer and it is difficult to reproduce the tests even for the same measurer. Therefore, an evaluation method based not on visual evaluation but quantitative data is strongly required for reliable and reproducible evaluation of scratch resistance.
Additionally, when the scratch resistance of a film or coating is evaluated using conventional methods, it is difficult to generate a scratch on the film or coating, and thus the evaluation of the scratch resistance may not be accurate.